Method for producing magnetostrictive element

ABSTRACT

It is an object of the present invention to provide a method for producing a magnetostrictive element which can give an anisotropic magnetostrictive element from a starting material containing Sm and a transition metal element. The method produces an anisotropic magnetostrictive element by compacting the starting powder in a magnetic field to produce a compact, and sintering the compact to produce a sintered body having a composition of SmFe 2 . It is preferable that the starting powder is produced by mixing a first alloy for the main phase of the magnetostrictive element with a second alloy having a lower melting point than the first alloy. It is recommended that the first alloy has a larger particle size than the second alloy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a magnetostrictive element, more particularly a method for producing a magnetostrictive element which contains Sm and a transition metal element.

2. Description of the Related Art

Magnetostriction is a phenomenon of a ferromagnetic material to undergo a dimensional change when magnetized, and a magnetostrictive material is a material which exhibits this phenomenon. Saturation magnetostrictive constant, which represents a dimensional change at saturation by magnetostriction, is generally in a range from 10⁻⁵ to 10⁻⁶, and a magnetostrictive material having a high saturation magnetostrictive constant is sometimes referred to as a giant-magnetostrictive material. These materials are widely used for vibrators, filters, sensors, and the like.

At present, a magnetostrictive material based on a laves type intermetallic compound of RFe₂, which is a compound of R (rare-earth element) and Fe, is known to have a high saturation magnetostrictive constant (refer to U.S. Pat. Nos. 3,949,351, 4,152,178, 4,308,474 and 4,375,372). These materials, however, involve problems of insufficient magnetostrictive value in an external magnetic field of low intensity, although exhibiting a high value when applied to a magnetic field of high intensity. Therefore, magnetostrictive materials based on a laves type intermetallic compound of RFe₂ have been studied to have a higher magnetostrictive value even in an external magnetic field of low intensity, and it is proposed to orient the material along the [111] axis as an easy-magnetization axis of high magnetostrictive constant. Magnetostrictive materials based on a laves type intermetallic compound of RFe₂ exhibit a high magnetostrictive value at a composition of Tb_(0.3)Dy_(0.7)Fe_(2.0) (atomic ratio), and this composition has been used exclusively.

SUMMARY OF THE INVENTION

An Sm—Fe-based material is one of the materials which exhibit a high magnetostrictive value at normal temperature, like Tb_(0.3)Dy_(0.7)Fe_(2.0).

A conventional magnetostrictive element of an Sm—Fe-based material is produced by melting a starting alloy and then solidifying the melt. However, the grains cannot be uniformly oriented by unidirectional solidification, because they pass the peritectic region of Fe₃Sm while being solidified. Therefore, the conventional process gives only an isotropic magnetostrictive element, and cannot give an anisotropic one.

The present invention has been developed to solve these technical problems. It is an object of the present invention to provide a method for producing a magnetostrictive element which can give an anisotropic magnetostrictive element, from a starting material containing Sm and a transition metal element such as an Sm—Fe-based material.

A method of the present invention for producing a magnetostrictive element, developed to achieve the above object, compacts a starting powder containing at least Sm and Fe into a compact in a magnetic field, and then sinters the compact to produce a sintered body having a composition of SmFe₂. This can give an anisotropic magnetostrictive element. The composition may contain trace quantities of at least one element selected from the group consisting of Ni, Co, Mo, W, Cr, Nb, Ta, Ti, V, Ru, Rh, Pt, Ag, Gd and B.

A powder of final composition may be directly used. It is however more preferable to use a mixed starting powder comprising a first alloy which forms the main phase for the magnetostrictive element and a second alloy which has a lower melting point than the first alloy. In the sintering process, the second alloy of lower melting point melts in advance of the first alloy to form the liquid phase. This can accelerate the sintering process. It is recommended that the starting powder contains the first alloy at 70% by weight or more but below 100% by weight.

It is also recommended that the first alloy has a larger mean particle size than the second one. This will improve degree of orientation of the first alloy, which forms the main phase, during the compaction in a magnetic field. In order to improve degree of orientation, it is recommended that the first alloy powder is mainly composed of single crystalline particles rather than polycrystalline particles. For production of the single crystalline particles for the first alloy powder, it is recommended that polycrystalline particles having a larger particle size than the second alloy are heat-treated beforehand to grow them into the single crystalline particles having a larger particle size than the second alloy. It is also effective that the second alloy powder is finely crushed by an adequate means, e.g., hydrogen crushing.

It is preferable to orient the starting powder in a magnetic field of 450 to 1760 kA/m in the compacting step.

The alloy having a composition of SmFe₂ has a lower magnetic anisotropy energy than an alloy composition of Tb_(0.4)Dy_(0.6)Fe_(2.0), for example. Therefore, an alloy composition of Tb_(0.4)Dy_(0.6)Fe_(2.0) is compacted in a magnetic field of 800 kA/m whereas an alloy composition of SmFe₂ needs a higher intensity of 1140 kA/m or so to have the same degree of orientation. By contrast, a mixed starting powder of first alloy and second alloy, the former having a larger particle size, can be oriented in a field of lower intensity than 800 kA/m, e.g., 760 kA/m in a compacting step.

The method of the present invention can produce an anisotropic magnetostrictive element by use of a starting material containing Sm and a transition metal element. Moreover, use of a mixed starting material of first and second alloys, the former having a larger particle size and the latter having lower melting point, allows orientation during compaction in a magnetic field and sintering to be effected efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 compares magnetic anisotropy energy of a magnetostrictive alloy material having a composition of SmFe₂ with that of a magnetostrictive alloy material having a composition of Tb_(0.4)Dy_(0.6)Fe₂; and

FIG. 2 illustrates the directions for applying a magnetic field and the directions for measurements, used to determine whether the sintered bodies prepared in Example are anisotropic or not.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described by the embodiments shown below.

The present invention uses a magnetostrictive material containing Sm and a transition metal element to produce an anisotropic giant-magnetostrictive material by powder metallurgy. In the above magnetostrictive material, Sm may be partly substituted by at least one element selected from the group consisting of Y, Nd and Tb.

A transition metal element T is at least one element preferably selected from the group consisting of Fe, Ni and Co, of which Fe is particularly preferable. A preferable magnetostrictive material is based on SmFe, accordingly. A transition metal element T may be partly substituted by at least one element M selected from the group consisting of Mo, W, Cr, Nb, Ta, Ti, V, Ru, Rh, Pt, Ag, Gd and B.

Moreover, an SmFe-based magnetostrictive material preferably has an SmFe₂ phase.

Such a magnetostrictive material can be produced by a process comprising a crushing, compacting in a magnetic field and sintering steps.

A starting material of final composition may be directly used, after being crushed. The embodiment of the present invention, however, adopts a characteristic method for producing a giant-magnetostrictive material in which a mixed starting powder comprising different compositions is used.

More specifically, 2 or more compositions of different melting point are used for the starting powder. In the embodiment, the mixed starting powder comprises a material for the main phase (hereinafter arbitrarily referred to as main phase material) and another material (hereinafter arbitrarily referred to as grain boundary phase material) having a lower melting point than the main phase material. When the mixed starting powder is sintered, the grain boundary phase material melts in advance of the main phase material to form the liquid phase. This can accelerate the sintering process to have a higher sintered body density. One of the preferable mixed powder compositions which satisfy the above conditions comprises an SmFe_(1.96) alloy (melting point: 900° C.) as the main phase material and Sm₂Fe alloy (melting point: 725° C.) as the grain boundary phase material.

The alloy powder of the main phase material is preferably single-crystalline, because of improved crystal orientation expected.

Moreover, the alloy powder of the main phase material preferably has a larger size than those for the grain boundary phase material, because they are difficult to be easily oriented while being compacted in a magnetic field when they are excessively small, and difficult to have a high sintered body density and hence deteriorate the reliability for the sintered body when they are excessively large.

In order to improve degree of orientation, it is recommended that the alloy powder for the main phase material is mainly composed of single crystalline particles rather than polycrystalline particles. It is recommended that polycrystalline particles having a larger particle size than the alloy powder for the grain boundary phase material are heat-treated beforehand to produce single crystalline particles having a larger particle size than the alloy powder for the grain boundary phase and use them as the alloy powder for the main phase material.

It is recommended that the mixed powder is heat-treated at 850 to 900° C. for 6 to 48 hours in a non-oxidative atmosphere, when alloy powder having a composition of SmFe_(1.96) is used for the main phase material, to have the single crystalline particles having a mean particle size (D50) of 12 to 30 μm.

The alloy powder for the grain boundary phase material is preferably crushed finely by being occluded with hydrogen, as required. When occluded with hydrogen, these alloy particles will be cracked and finely crushed due to internal stresses caused by strains produced as a result of formation of hydride or penetration of atomic hydrogen into the grains. When the alloy powder having a composition of Sm₂Fe (melting point: 725° C.) is used for the grain boundary phase material and occluded with hydrogen, the alloy powder is crushed to have a mean particle size of 5 to 20 μm decreased from around 20 μm.

The mixing ratio of the main phase material to the grain boundary phase material may be set arbitrarily, but preferably according to the following relationship.

The content “a” (% by weight) of the main phase material is preferably in a range of 70≦a<100, more preferably 80≦a≦95. When it is excessively low, i.e., ratio of the main phase material to be oriented in a magnetic field is excessively low, degree of orientation after sintering may be insufficient. When it is excessively high, on the other hand, by which is meant that the main phase material composition is close to the final composition, it signifies little to use a grain boundary phase material for improved sintered body density.

The magnetostrictive material produced by weighing and mixing the main phase material and grain boundary phase material may be then crushed. In the crushing step, a crushing machine may be adequately selected from a wet ball mill, attritor, atomizer and the like. Of these machines, an atomizer is particularly preferable, because it can apply an impact and shear stress simultaneously to the particles to prevent their agglomeration and hence enhance productivity. The crushing is preferably conducted in a non-oxidative atmosphere, e.g., in an inert gas atmosphere of Ar gas or the like, or under vacuum. The mean particle size after crushing is 5 to 20 μm, preferably 10 to 20 μm. When it is excessively small, the particles tend to be oxidized in the production process to deteriorate the magnetostrictive characteristics. When it is excessively large, on the other hand, the sintered body will have an insufficient density and a number of open pores, because of insufficient sintering rate.

The mixed magnetostrictive material is compacted into a desired shape before sintering. The compacting is carried out in a magnetic field to principally orient the main phase material particles in one direction. This is to orient the sintered magnetostrictive material along the [111] axis. The magnetic field applied has an intensity of 480 to 1760 kA/m, preferably 960 to 1760 kA/m. Magnetic field direction may be perpendicular to or in parallel to pressure direction. Compacting pressure is 50×10⁶ Pa or more, preferably 300×10⁶ Pa or more. A magnetostrictive alloy material represented by SmFe₂ (Formula (1)) has a magnetic anisotropy energy R roughly half that of a composition of Tb_(0.4)Dy_(0.6)Fe_(2.0), for example, as shown in FIG. 1. This means that SmFe₂ needs a higher magnetic field intensity than Tb_(0.4)Dy_(0.6)Fe_(2.0) to improve degree of orientation during the compacting in a magnetic field.

The compact produced by compacting in a magnetic field is then sintered. The recommended sintering conditions are 800 to 900° C., preferably 850 to 890° C., and 3 to 48 hours. The recommended sintering atmosphere is non-oxidative, preferably in an inert gas atmosphere, e.g., in an Ar gas, or under vacuum.

The magnetostrictive material thus prepared is polycrystalline and represented by SmFe₂ (Formula (1)). It is oriented along the [111] axis, a direction in which it exhibits the highest magnetostriction.

As described above, the method of the present invention can produce an anisotropic giant-magnetostrictive material by powder metallurgy from a starting magnetostrictive material containing Sm and a transition metal element.

EXAMPLES

It is confirmed in this example that an anisotropic giant-magnetostrictive material can be produced by powder metallurgy from a starting magnetostrictive material containing Sm and a transition metal element. The results are described below.

First, a sintered body as a magnetostrictive element main body was produced by the following procedure.

First, Sm and Fe as the main phase materials were weighed and molten in an inert Ar gas atmosphere to have a starting alloy having a composition of SmFe_(1.96). The starting alloy was heat-treated by annealing at 890° C. for 12 hours after temperature was stabilized to grow the grains. The obtained alloy powder was passed through a sieve (opening size: 2 mm) to remove coarse particles of 2 mm or more.

Sm and Fe as the grain boundary phase materials were weighed and molten in an inert Ar gas atmosphere to have a starting alloy having a composition of Sm_(2.0)Fe. The starting alloy powder was heat-treated at 150° C. for 6 hours after temperature was stabilized in a hydrogen atmosphere (hydrogen concentration: 80%) to crush the alloy occluded with hydrogen at around 18,000 ppm. The resulting crushed powder had a mean particle size of 5 μm. The crushed powder was passed through a sieve (opening size: 2 mm) to remove coarse particles of 2 mm or more.

Then, the alloy powder thus produced for the main phase and grain boundary phase materials were weighed and mixed with each other. The resulting mixture was finely crushed in an Ar gas atmosphere by an atomizer to have an alloy powder having a composition of SmFe_(1.875).

The alloy powder was put in a mold and compacted in a magnetic field of 480 kA/m (6 kOe) at a compacting pressure of 8 tons/cm² to have a compact. It was transferred into the mold via a pipe filled with N₂ gas. The magnetic field was the so-called transverse one, in which the magnetic field was applied in a direction perpendicular to pressure direction. The compact (prepared in Example) was 10 mm cubic.

For comparison, the alloy powder was put in a mold and compacted under the same compacting pressure of 8 tons/cm², but in the absence of a magnetic field, to have a compact having the same shape and size as the above (Comparative Example 1).

Each of the compacts was put in a container for sintering and heated in a furnace in an Ar gas atmosphere at 890° C. for 6 hours after temperature was stabilized to have a sintered body (magnetostrictive element main body). The sintered body thus prepared was polycrystalline, represented by SmFe₂, and was oriented along the [111] axis.

For comparison, the starting alloy having a composition of SmFe_(1.90) was molten at 900° C., and the melt was solidified in a mold to have a magnetostrictive element of the same shape and size as the above by casting (Comparative Example 2).

Each of the sintered bodies prepared in Example and Comparative Examples 1 and 2 was measured for the magnetostrictive characteristic (magnetostrictive value). Those prepared in Example and Comparative Example 1 were measured for sintered body density.

The magnetostrictive value was determined by measuring the elongation of the samples (sintered bodies) using a strain gauge while the samples were placed in a magnetic field of 80 kA/m (1 kOe), where a magnetic field used for measurement was applied in the direction in parallel to the magnetic field direction applied during compaction (X-direction) and also perpendicular to the magnetic field direction applied during compaction (Y-direction). FIG. 2 illustrates elongation in the X- or Y-direction when the samples were subjected to a magnetic field used for measurement in the X- or Y-direction. The results are given in Table 1. TABLE 1 Magnetostrictive value Magnetic field direction used for measurement X Y Elongation Elongation Sintered measurement measurement body direction direction density (%) X Y Y X Example 96.34 −475 250 −275 145 Comparative 96.30 −275 65 −245 135 Example 1 Comparative — −260 90 −250 85 Example 2

As shown in Table 1, the samples prepared in Comparative Examples 1 and 2 had an elongation of −275 and −260, respectively, in a direction along the magnetic field direction used for measurement (X-direction), when the magnetic field was set in the X-direction. They had an elongation of −245 and −250, respectively, in a direction along the magnetic field direction used for measurement (Y-direction), when the magnetic field was set in the Y-direction. By contrast, the sample prepared in Example had an elongation of −475 in a direction along the magnetic field direction used for measurement (X-direction), when the magnetic field was set in the X-direction, and −275 in a direction along the magnetic field direction used for measurement (Y-direction), when the magnetic field was set in the Y-direction. These results indicate that the sample prepared in Example had a magnetrostrictive value widely varying depending on direction of magnetic field applied thereto, i.e., it was anisotropic.

It is therefore confirmed that the method of the present invention can produce an anisotropic magnetostrictive element. 

1. A method for producing an anisotropic magnetostrictive element, comprising the steps of: compacting a starting powder containing at least Sm and Fe into a compact in a magnetic field, and sintering the compact to produce a sintered body having a composition of SmFe₂.
 2. The method for producing a magnetostrictive element according to claim 1, wherein the starting powder is produced by mixing a first alloy which forms the main phase of the magnetostrictive element with a second alloy which has a lower melting point than the first alloy.
 3. The method for producing a magnetostrictive element according to claim 2, wherein the first alloy has a composition of SmFe_(1.96).
 4. The method for producing a magnetostrictive element according to claim 2 or 3, wherein the second alloy has a composition of Sm_(2.0)Fe.
 5. The method for producing a magnetostrictive element according to claim 2, wherein the starting powder contains the first alloy at 70% by weight or more but below 100% by weight.
 6. The method for producing a magnetostrictive element according to claim 2, wherein the starting powder contains the first alloy between 80 and 95% by weight.
 7. The method for producing a magnetostrictive element according to claim 2, wherein the first alloy powder has a larger particle size than the second alloy powder.
 8. The method for producing a magnetostrictive element according to claim 7, wherein the first alloy powder has a mean particle size of 12 to 30 μm.
 9. The method for producing a magnetostrictive element according to claim 7, wherein the first alloy powder is mainly composed of single crystalline particles and heat-treated beforehand to have a larger particle size than the second alloy powder.
 10. The method for producing a magnetostrictive element according to claim 9, wherein the first alloy powder is heat-treated at 850 to 900° C. for 6 to 48 hours, in a non-oxidative atmosphere.
 11. The method for producing a magnetostrictive element according to claim 1, wherein the compact is sintered at 800 to 900° C. for 3 to 48 hours, in a non-oxidative atmosphere.
 12. The method for producing a magnetostrictive element according to any of claim 1, wherein the starting powder is oriented in a magnetic field of 450 to 1760 kA/m in the step of producing a compact.
 13. A method for producing an anisotropic magnetostrictive element, comprising the steps of: mixing a first alloy powder which has a composition of SmFe_(1.69) with a second alloy powder which has a composition of Sm_(2.0)Fe to prepare a starting powder, compacting the starting powder into a compact in a magnetic field, and sintering the compact to produce a sintered body having a composition of SmFe₂.
 14. The method for producing a magnetostrictive element according to claim 13, wherein the starting powder contains the first alloy between 80 and 95% by weight.
 15. The method for producing a magnetostrictive element according to claim 13, wherein the first alloy powder has a mean particle size of 12 to 30 μm, and the second alloy powder has a smaller particle size than the first alloy powder.
 16. The method for producing a magnetostrictive element according to claim 15, wherein the first alloy powder is mainly composed of single crystalline particles.
 17. The method for producing a magnetostrictive element according to claim 13, wherein the sintered body is polycrystalline and the grains of the sintered body are oriented along the [111] axis. 